Magnetic bearings operate without mechanical friction, but require continuous supply with electrical power. In case of a power failure, the shaft needs to be supported by a so-called auxiliary bearing, which is also called landing bearing, emergency bearing or back-up bearing.
Thus rotary machines equipped with active magnetic bearings include mechanical auxiliary bearings which act if one of the magnetic bearings is overloaded or if the electrical or electronic control circuit fails or else in the event of stoppage or of suspension overload.
An auxiliary device provides mechanical redundancy and needs to be defined and dimensioned correctly so as to fully guarantee that the machine will not be damaged and that the equipment can recover immediately once the overload or servo-control failure has disappeared.
Rolling element bearings which are generally dry-lubricated or sleeve-bushing combinations (smooth rings) can be used for constituting auxiliary bearings.
More specifically auxiliary bearings using rolling element bearings may use ceramic rolling element bearings. However such bearings are very costly and moreover require a lot of space.
Auxiliary plain bearings having smooth surfaces, in particular auxiliary bearings using sleeve-bushing combinations have various drawbacks. In particular sleeves have typically insufficient load carrying capacity and cannot handle high sliding speeds. Due to the air gap of the magnetic bearing, it cannot be predicted if the shaft will drop in such a way that it is axially aligned with the center axis of the bushing. It could happen that the shaft and the bushing are misaligned instead of being coaxial. This creates very high contact pressures on the edges of the sleeve which is mounted on the shaft. Therefore, the use of sleeves is often avoided.
Rotors mounted on magnetic bearings often present nominal speeds which are very high. Under such circumstances, in the event of the rotor landing on the auxiliary bearings due to a failure of control or power supply, the rotor presents whirling movements at its speed of rotation with eccentricity which is then defined by the clearance of the auxiliary bearing. Under such circumstances, a very high degree of unbalance can arise leading to destruction of the bearings or to deformation of the rotor.
FIG. 1 shows an example of a conventional radial magnetic bearing 12 for supporting a rotating shaft 10. The magnetic bearing 12 comprises a stator constituting an electromagnet and including a ferromagnetic core 14 and coils 13. An air gap E1 is defined between the stator 13, 14 and the peripheral outer surface of the shaft 10, which constitutes a rotor armature.
An auxiliary bearing 18 of the sleeve-bushing type comprises a sleeve 20 which may be for example press-fitted on a reduced diameter cylindrical part 22 of the shaft 10. A bushing 21 is mounted in a housing 16 and has a cylindrically-shaped layer secured to the housing 16 and being coaxially disposed with respect to the sleeve 20. A clearance E2 is defined between an inner cylindrical surface 24 of the bushing 21 and an outer cylindrical surface 23 of the sleeve 20.
The clearance E2 between the opposing surfaces 23, 24 of the auxiliary bearing 18 may be for example between 0.2 and 0.3 millimeter and is smaller than the air gap E1 of the magnetic bearing 12, which may be for example between 0.4 and 0.6 millimeter. Usually the clearance E2 is about half the air gap E1.
In the conventional auxiliary bearings of the sleeve-bushing type such as the auxiliary bearing 18, the opposing surfaces 23, 24 define a rectilinear profile in a longitudinal axial cross-section as shown in FIG. 1. As mentioned above, when there is a failure of the magnetic bearing 12 and the shaft 10 with its sleeve 20 lands on the bushing 21, if during this landing the shaft is misaligned with the bushing, very high contact pressures are created on the edges of the sleeve.